Studies are continuing on the process of recognition of foreign antigen by human T cells, particularly with respect to the role of T cell surface molecules in cytotoxic T cell (CTL) interaction. Recognition of the SB antigens has been used as a model system. In order to analyze functional heterogeneity among CTL clones a panel of SB2-specific CTL clones has been derived. Analysis by monoclonal antibody blocking demonstrates that susceptibility to inhibition by antibodies against some T cell surface markers (anti-T3 and anti-T4) varies markedly from one clone to another; in contrast, inhibition varies little or none with other antibodies (anti-T11 and anti-LFA-1). Clonal 'avidity', inferred from analysis of the capacity of competitor cells to disocciate preformed effector-target cell conjugates, correlates with clonal susceptibility to inhibition by both anti-T3 and anti-T4. This correlation was confirmed for anti-T4 but not for anti-T3 when inhibition was compared for the same CTL clone on: a) a SB2-positive target and b) a cell line which expressed a 'crossreactive' specificity with which the effector interacted with lower avidity. Partitioning of the assay into conjugate formation vs delivery of the lethal hit demonstrated that anti-T4 the former and that anti-T3 inhibits the latter. Thus, the T4 molecule may be functionally involved principally in conjugate formation by clones of relatively low avidity while there must be a different biological basis for the heterorgeneity of anti-T3 inhibition.